1. Technical Field
The subject of the present invention is a transport device for transporting sheet elements of low specific mass, made of paper or cardboard, into a folder-gluer which is a machine commonly used in the packaging industry for making up cardboard boxes for example.
2. Prior Art
Traditionally, a folder-gluer comprises a series of modules and stations the number of which varies according to the complexity of the manufacturing operations that the type of box chosen entails. Such machines generally consist of a sheet feeder feeding the box production line blank by blank from a stack, an alignment module, a breaker that pre-breaks the 1st and 3rd folds between 90° and 180°, and a fold hook module, which folds the front flaps and then the rear flaps of the blank at 180°, a gluing station, a folder for folding the 2nd and 4th folds of the blank, a press which compresses the 2nd and 4th folds and sets down the boxes in a layer, and finally, a receiving module which receives the boxes keeping them pressed firmly in order to allow the glue to dry. The blanks are conveyed from one station to the next using belt-type conveyors which, through friction, grip the blanks between a bottom conveyor and a top conveyor. Traditionally, the bottom conveyor is equipped with bottom belts and the top conveyor is equipped either with top belts or with top pressing rollers.
The bottom conveyor comprises two or more longitudinal members each supporting an endless conveyor belt supported by pulleys and rollers. Each longitudinal member is mounted with the possibility of lateral sliding via bearings along one or more movement slides mounted fixedly between two longitudinal support structures. In order to adapt the lateral position of the longitudinal members to suit the format of the blanks to be processed, each longitudinal member can be moved laterally by one or more parallel screws mounted such that they can rotate between the supporting structures, the threaded portions of the screws being engaged in respective transverse tapped orifices belonging to the longitudinal members.
For each longitudinal member, the endless conveyor belt is driven via a drive shaft mounted in rotation between the supporting structures and engaged in a pulley belonging to the longitudinal member, termed the drive pulley. The drive shaft is connected by a drive line to an electric motor and so when the motor turns, the shaft is rotated.
In general, the drive shaft is in the form of a metal bar of polygonal, for example hexagonal, cross section collaborating in terms of shape with a transverse orifice formed along the axis of the drive pulley. Thus, when a longitudinal member moves laterally under the action of the adjusting screws, it slides along movement slides and along the drive shaft.
It will be noted that the power needed to operate and to drive the blanks through the machine is dependent on the width of the machine. As a result, the wider the machine the greater the forces that have to be transmitted, this meaning that the torsional strength of the shaft has to be increased, for example by increasing its cross section. Thus, the drive shaft ought to be designed merely to withstand torsion because the weight of the longitudinal members is borne by the movement slides, although in practice it is found that machine operators do not hesitate to climb onto the drive shaft in order to access certain parts of the machine. Hence, in order to prevent the drive shaft from buckling, this drive shaft is also designed to withstand the weight of a person without deflection, thus increasing the cost of manufacture of the shaft.
In addition, for safety reasons, parts of the drive shaft that are exposed to the machine operator are engaged in sleeves. A sleeve is generally in the form of a volute spring one end of which is secured to a supporting structure and the other end of which is secured to a longitudinal member, the lateral movement of the longitudinal member extending or compressing the spring. It will be readily understood that the presence of a spring such as this means that the lead screws that drive the longitudinal members have to be sized accordingly. This is because the greater the stiffness of the volute spring, the more torque will have to be exerted on the screws in order to be able to move the longitudinal members, this also entailing adapting the electric power of the motors that turn the screws accordingly.